Just in case anyone has a similar situation, here's what happened to me switching from 30 mg Prevacid to Prilosec 20mg.
After 1 day, I started getting heartburn, day or night. After 3 days, nausea came back and lots of heartburn, almost to pains in chest. Day 4 took a 2nd 20mg in evening. Better. Started taking (2) 20mg prilosec each day for 3 weeks. Eating ice cream or chocolate gives lump in throat, heartburn or nausea. Mornings have nausea until taking the (2) 20mg prilosecs. Last day took (2) 20mg's in morning and (1) 20mg in eve. Still have nausea in morning.
Have some Prevacid 30mg's left. Went back on them and all is ok.
It appears at least for me, you don't have to wait 7-10 days for the Prilosec to start working if you're already using Prevacid and when going back to it.
Don't know if using (2) 20mg prilosec would work the same as (1) Omaprazole 40mg.
Also, noticed insurance will only give a $10 copay for Omaprazole 40mg while a $35 copay for Prevacid 30mg.
Hope this helps someone else who finds themselves in a similar situation.